For synthesis of an organic compound, for example in a case that reaction is inhibited by a hydroxy group, a case that the hydroxy group itself reacts, a case that the water solubility due to a hydroxy group of sugar or the like is lowered and the compound is rendered liposoluble, etc., the hydroxy group should be protected, and therefore various protecting groups have been used. A protecting group is a functional group for temporarily inducing a functional group disturbing synthetic reaction to be structurally inactive (protecting), and restoring it by desorption after reaction.
As the protecting group, various groups are known, and above all, benzyl group is very frequently used for reasons that it is considerably stable and hardly broken, that it is easy to eliminate by catalytic reduction (hydrogenation) using a metal of palladium or the like, that the deprotected compound can be easily purified (filtered and vaporized) because the compound becomes toluene after the elimination, etc.
As a method for introducing the benzyl group, a method using Williamson's ether synthesis as shown in Formula 1 is known, in which the reaction requires a basic condition, and cannot be applied to protection of alcohols comprising a functional group which is destabilized or decomposed in a basic condition in their molecules. In addition, a halogenated benzyl used as a hydroxy group-protecting agent (protecting agent) is carcinogenic and lachrymatory and thus undesirable in health or the like.

For benzylating reaction using a halogenated benzyl, use of argentate is also considered, and for example, use of silver oxide (Ag2O) as a catalyst allows the hydroxy group to be benzylated and protected even if not under a basic condition, as shown in Formula 2. However, silver oxide is expensive and has a great cost problem.

Meanwhile, for the introduction of the benzyl group under an acidic condition, for example a method using benzyltrichloroacetimidate as shown in Formula 3 has been proposed. However, said benzyltrichloroacetimidate has disadvantages that it is easy to hydrolyze and susceptible to heat and moisture, thus handling is troublesome, and its price is high.

From such situations, methods in which the hydroxy group can be stably protected under an acidic condition have been studied. For example, protection of the hydroxy group by an acetal-type protecting group is proposed in Patent Document 1. Patent Document 1 describes that an acetonitrile reagent used for manufacturing medicines, agrochemical, etc. is used as a protecting agent, and a protecting group can be efficiently introduced into the hydroxy group under a mild reaction condition like slight acidity.